We added a small percentage of granular manganese to provide dark speckling in the body. That speckle bleeds through most glazes. As manganese is gaseous at higher temperatures, if you fire in reduction we recommend a lighter reducing atmosphere and slightly cooler firing to avoid trapped surface gases that can cause bubbling.
Fires Between Cone 6-9, 2232-2300° F
Cone 6 - 13%, Cone 9 - 14%
Cone 6 - 4.1%, Cone 9 - 2.6%